It has been established that the occurrence of gold in rocks and veins presents even greater peculiarities and varieties of form than those of the alluvial deposits. The following are some of the forms in which it is found: crystals, specks, flakes, spangles, filaments, branches, wires, threads, leaf-like and sponge like forms, irregularly shaped lumps of all sizes, from dust-like particles to that which in alluvial gold would be considered large nugget sizes; and there is known to exist even a finer division than that which may be termed dust; for in certain cases, where microscopic examination showed no gold, considerable quantities have been obtained by chemical treatment.
It is now beyond doubt or argument that by far the greater wealth of gold ore deposits at present known are those wherein the metal exists in finely divided particles disseminated throughout vein structures and deposits. In like manner a very general occurrence of gold is in association with the sulphides and arsenical compounds of iron, and the sulphides of copper, lead, zinc, antimony, and bismuth. The prospector as a rule classes any mixture of the above under the general term mundic or pyrites. Although by some it is considered a debatable question as to whether gold is chemically combined or mechanically held in these compounds, the balance of evidence is strongly in favor of the latter condition, and to this view most of the leading metallurgists of the day are inclined. When so occurring it is indisputable that in the majority of cases gold is in a state of extreme division, so much, that to describe it as flour would be a poor illustration of its finely divided condition.
There are certain large auriferous districts where the metal is found almost entirely associated with the above-mentioned compounds, but these districts are not the sole localities wherein it is so deposited; in gold-producing districts where the veins carry free gold, patches of gold-bearing sulphides are frequently met with; and in many localities free gold and gold-bearing sulphides are very evenly mixed throughout the deposits. The popular and eagerly sought auriferous deposits are those in which gold is here and there visible in irregularly shaped particles, though such an occurrence by no means denotes richer deposits than those carrying invisible gold or gold-bearing compounds; indeed, it is frequently found that the latter are more dependable in their return of metal. There are numberless instances of the occurrence in veins of heavy gold-bearing patches of stone of great value, where the metal was in large and irregularly shaped particles, sometimes in flakes and plates, like ham in a pile of sandwiches.
One of the most valuable of these discoveries was in the Hill End Mine, near Sydney, New South Wales. The patch of stone in question was about 1.5 m. high, and on an average 0.30 m wide and 0.15 m thick, it was full of gold in the form of threads, wires, lumps, and irregularly shaped particles, and its value was very high. Another form in which gold occurs is that in the world-famous Mount Morgan Mine, near Rockhampton, Queensland. The deposit presented several interesting features, as may be seen from the following brief description, the mountain, situated about 35 km from the sea, is surrounded by a tableland. The country rock consisted of a sandstone formation, with occasional masses of hard shale, and in the immediate locality of the mountain it is intersected in every direction with intrusive masses of igneous rocks, such as rhyolite and dolerite.